What Is a Sequence and Why Use It

A sequence is a set of operations carried out in a defined order. A controller remembers which step is active, performs that step's work, and moves on when the required condition is met.

Think of a washing machine: wash, rinse, then spin. Each phase has a different job, and the machine needs to know which phase it is in before deciding what to do next.

What you will learn

• Explain a sequence as steps with conditions for moving between them.

• Distinguish the active step from the programmable logic controller (PLC)'s repeating scan.

• Describe why startup, stopping, and faults need an explicit place in the plan.

A machine that does things in order

A packaging station must fill a container, seal it, then release it. Starting all three operations together would not perform that job. The controller must remember how far the current container has progressed.

We can describe the plan before choosing a programming language:

Idle → Fill → Seal → Release → Idle

The active step is the station's state: the part of the job it is doing now. A transition is the change from one state to another when a condition is satisfied.

A step can last for many scans

The PLC keeps scanning while the station fills. On each scan, the program checks the current step and the relevant inputs. It stays in Fill until filling is complete; a new scan by itself does not mean a new step.

This also explains why an input's meaning depends on the active step. The filling-complete signal matters when leaving Fill. It does not tell the station to start sealing again after the container has already reached Release.

Plan the transitions as carefully as the actions

For each step, ask what should happen here, what proves it is finished, and where should the sequence go next? Completion might come from a sensor, a timer, a count, or another part of the machine.

Also decide what happens when the expected condition never arrives. A timeout can stop normal progression and move the machine into a defined fault state. A condition saying that operation is permitted is different from evidence that the step actually finished.

Decide how the cycle starts and stops

Give the sequence a known starting state. Define which actions stop when an operator presses Stop, what happens to the active step, and what a later Start or Reset is allowed to do.

Restarting a cycle from Idle and resuming an interrupted step are different operating rules. Choose the intended rule explicitly so the machine does not resume merely because an old step value remained in memory.

Check the plan before writing code

Suppose the station is in Seal, and its filling-complete signal becomes FALSE. Should that alone send it back to Fill? Use the table to explain your answer.

Check your explanation

No. The stated Seal step waits for sealing to complete, then moves to Release. The plan contains no transition from Seal back to Fill based on that signal. A separate fault or operating requirement would need its own defined behavior.

What comes next

You can implement this kind of plan in Ladder or Structured Text. The program needs a way to store the active step, select its actions, and make the intended transitions.

Next, Sequencing in Ladder builds a small batch sequence and compares a numbered step with one bit per step. It also puts stopping, timeouts, and reset behavior into the program.

Enable JavaScript to run the embedded simulator and complete the interactive exercises.